/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
 //快慢指针
class Solution {
public:
    bool hasCycle(ListNode *head) {
        if(head==nullptr || head->next==nullptr)
            return false;
        ListNode* fast=head;
        ListNode* slow = head;
        while(fast) {
            if(fast->next != nullptr)
                fast = fast->next->next;
            else
                return false;
            slow = slow->next;
            if(fast == slow)
                return true;

        }
        return false;
    }
};